The influence of ultrasonic vibration on parts properties during incremental sheet forming

The integration of ultrasonic vibration into sheet forming process can significantly reduce the forming force and bring benefits including the enhancement of surface quality, the enhancement of formability and the reduction of spring-back. However, the influencing mechanisms of the high-frequency vibration on parts properties during the incremental sheet forming (ISF) process are not well known, preventing a more efficient forming system. This paper comprehensively investigates the effects of different process parameters (vibration amplitude, step-down size, rotation speed and forming angle) on the micro-hardness, minimum thickness, forming limit and residual stress of the formed parts. First, a series of truncated pyramids were formed with an experimental platform designed for the ultrasonic-assisted incremental sheet forming. Then, micro-hardness tests, minimum thickness measurements and residual stress tests were performed for the formed parts. The results showed that the surface micro-hardness of the formed part was reduced since the vibration stress induced by the ultrasonic vibration within the material which eliminated the original internal stress. The superimposed ultrasonic vibration can effectively uniform the residual stress and thickness distribution, and improve the forming limit in the case of the small deformation rate. In addition, through the tensile fracture analysis of the formed part, it is shown that the elongation of material is improved and the elastic modulus and hardening index are decreased. The findings of the present work lay the foundation for a better integration of the ultrasonic vibration system into the incremental sheet forming process.